Rechargeable galvanic cells, with which this invention is concerned comprise a cathode, a zinc anode, a separator having at least one layer of a semipermeable membrane and an aqueous alkaline electrolyte, such as an aqueous solution of potassium hydroxide. The cathode may comprise manganese dioxide, bismuth modified manganese oxides, silver oxide, nickel oxyhydroxides or an air electrode. Graphite or carbon black is admixed to the cathode active materials to impart electronic conductivity, while potassium hydroxide is admixed to provide the necessary ionic conductivity to the cathode. The zinc anode mixture, will include zinc as one of the main constituents, and will also include electrolyte and other constituents in known manner. These cells display superior electrical performance, in particular at high discharge rates or at low temperatures, and are widely used in many applications.
In light of environmental concerns surrounding the disposal of batteries, toxic materials utilized as additives in manganese dioxide/zinc cells such as mercury are being drastically reduced or eliminated from the cell chemistry. A problem common to zinc electrodes is that zinc is corroded by most aqueous electrolytes resulting in generation of hydrogen gas. Traditionally, zinc powder has been amalgamated with mercury to elevate the hydrogen overpotential thereby suppressing gassing and leakage problems. Mercury additions have also provided an additional benefit in constituting a conductive additive resulting in superior electrical performance of galvanic cells utilizing amalgamated zinc as negative electrode materials, in particular at high discharge rates, at low temperatures and under conditions where the cells are exposed to shock and vibration.
Due to these increasing environmental concerns it has become desirable to reduce or eliminate the amount of mercury used in galvanic cells. Generally, in primary cells, methods have focused on the use of selected metals, and the use of organic corrosion inhibitors, to prevent hydrogen generation within the galvanic cell. In rechargeable cells, the detrimental growth of dendrites caused by the recharge processes must be prevented in addition to reducing the hydrogen evolved by corrosion and during recharge.
The employment of low surface tension metals such as lead, indium, gallium, thallium, bismuth, calcium and aluminium is disclosed in the technical literature. The metals are added as minor alloying agents. Substantial prior art exists describing alloys of zinc with indium with and without mercury and other alloying agents. In the case of rechargeable cells containing a zinc negative electrode the cycle life benefits from employing indium or its compounds as dendrite preventers are known. This has been documented in the patent and other literature since around 1960. Similarly, the use of nonionic and anionic surfactants as dendrite preventers is also described from around the same time.
Similarly, the known art since around 1960 addresses the problem of the surface coating of the zinc powders with appropriate metals or their compounds, prior to processing the negative electrode. However, the techniques were often complicated and frequently included filtering, washing and drying steps. The present inventors have discovered that the washing and drying steps alter the surface coating on the zinc powder, containing the dendrite and corrosion preventers, so that they become less effective.
Adding indium during the negative electrode assembly procedure either in form of indium compounds or alternatively dissolving appropriate metal compounds in the cell's electrolyte is also described in the prior art.
The use of organic surfactants and indium for prevention of zinc dendrite growth and hydrogen evolution has been known for over thirty years. A number of commercially available surfactants can raise the hydrogen overpotential of metallic zinc or zinc alloys which can optionally contain a surface coating of a metal. It has also been reported that a number of effective organic surfactants when used in rechargeable galvanic element containing a zinc negative electrode provide an additional benefit in terms of the cell capacity and cycle life.
H.M. Kiel in DE 1,086,309 (1961) is one of the oldest proposals and introduces art relating to zinc-indium alloys, as well as indium ions in the electrolyte. Kiel describes a primary or secondary galvanic cell with a zinc electrode in an acidic, neutral or alkaline electrolyte, characterized by the addition of indium compounds to the electrolyte or alloying indium with high purity zinc. It describes in great detail the self discharge of zinc active material in acidic, neutral and alkaline electrolytes resulting in the liberation of hydrogen gas, cell leakage and the resulting limited shelf life of the respective cells. In the claims the patent describes indium additions to a galvanic cell containing a zinc negative electrode, either in the form of a zinc-indium alloy or, alternatively addition of In compounds to the electrolyte, in which the zinc has a purity of 99.99%, and the use of an alkaline, acidic or neutral electrolyte.
A. Kawakami in U.S. Pat. No. 3,642,539 (1972) adds an indium compound to the cell bottom, the separator or the electrolyte, to prevent dendrite or spongy zinc in rechargeable zinc air cells. H. Ikeda in Japanese published application J6032363 (1976) treats zinc powder in an acidic indium chloride solution and then filters, washes and dries the zinc powder.
Canadian patent No. 1,267,189 to Winger describes a primary or single use galvanic cell having a manganese dioxide cathode, an alkaline electrolyte solution and a zinc anode containing mercury. Winger states that the amount of mercury used in the cells can be reduced to between about 0.04% and about 3.0 weight percent based on the weight of the zinc by incorporating both a compound containing polyethylene oxide linkages and indium in the cell. However, Winger states that the use of mercury cannot be completely eliminated, as if the amount of mercury is below 0.04% by weight, storage stability is adversely affected even with the addition of both a compound having polyethylene oxide linkages and indium.
U.S. Pat. No. 5,198,315 (K. Tada) apparently discloses a primary zinc alkaline cell which uses non-amalgamated zinc alloy powder as an anode active substance. The zinc alloy powder is surface coated with indium and has a bulk specific gravity adjusted to range from 2.90 to 3.50 grams per cubic centimetre. Two methods of coating the zinc alloy with indium particles are described. The first method involves charging a heated mixer with a predetermined amount of zinc alloy powder and a predetermined amount of indium particles and nitrogen gas, and mixing at 180.degree. C. for one hour. The second method involves mixing a predetermined amount of zinc alloy powder with a predetermined amount of an indium salt, such as indium sulphate in water and stirring for 30 minutes. The resulting zinc alloy powder was filtered, washed with purified water, had the water adhering on the zinc alloy powder replaced by acetone and then dried at 45.degree. C. for one day.
U.S. Pat. No. 5,168,018 to Yoshizawa discloses a method of manufacturing a mercury free zinc alkaline battery in which the anode comprises zinc alloy powder as an active material and contains an indium hydroxide powder dispersed therein and an organic corrosion inhibitor, such as perfluoroalkyl polyethylene oxide surfactant. The indium hydroxide powder used is preferably synthesized by neutralizing an aqueous solution of indium chloride or indium sulphate. It is indicated that indium chloride is preferred, as indium hydroxide powder based on indium chloride has a better corrosion resistance than when indium sulphate is used. When the indium hydroxide powder is dispersed with the zinc alloy powder into electrolyte, part of the indium hydroxide is electrodeposited onto the surface of the zinc alloy, and part is retained in a solid form in the electrolyte, for electrodeposition onto fresh zinc surfaces exposed during discharging. Again, Yoshizawa's disclosure relates to single use, primary cells.
More recently, there have been a number of proposals for alkaline cell systems configured for use as secondary or rechargeable cells.
K. Kordesch et. al. in U.S. Pat. No. 4,925,747 (1990) describes a primary or rechargeable electrochemical cell such as an alkaline manganese dioxide-zinc primary or secondary cell in which hydrogen is recombined between 5 and 15 psig up to the relief pressure of the cell by incorporating an auxiliary electrode or electrode material and a catalyst for the absorption of hydrogen in presence of electrolyte and in intimate electrical contact with the cathode. Auxiliary electrodes described are noble metal porous gas diffusion electrodes placed on the top of the cathode which oxidize hydrogen in presence of electrolyte and the electrocatalyst to water. Noble or non noble metal catalysts admixed into part of the entire cathode were found to be useful in recombining hydrogen gas as well. In the case of silver catalyst the amount of silver required was relatively high (3-30%Ag.sub.2 O). It also mentions nickel and nickel alloys with lanthanum or titanium as possible catalysts.
K. Tomantschger et. al. in U.S. Pat. No. 5,162,169 (1992) demonstrated hydrogen recombination utilizing only small amounts of silver, silver compounds and noble metals in conjunction with MnO.sub.2. Again, as in U.S. Pat. No. 4,925,747 intimate electronic and ionic contact is maintained between the hydrogen recombination catalyst and the cathode. In fact, it was demonstrated that the reduction of the silver content to or below 0.1% of the cathode (125 ppm of the Ag/MnO.sub.2 mixture) still resulted in appreciable hydrogen recombination rates. The recombination does not necessarily require any overpressure to proceed. The hydrogen recombination catalyst represents between 0.1 and 30% of the weight of the catalyst/MnO.sub.2 mixture and may be supported on a porous substrate such as carbon or graphite. The auxiliary electrode material can be included in part or in the entire cathode.
F. Parsen in U.S. Pat. No. 4,350,745 (1982) describes an agent for absorbing hydrogen gas made of MnO.sub.2 and non noble metal catalysts such as nickel, cobalt or iron. The recombining article is preferably formed into small agglomerates which can be added to the cathode and/or electrolyte of the cell or combined with the material to be formed into the cathode or anode preferably in the form of small capsules (e.g. microballoons). Oxides of manganese or lead form the absorbent and a powdered metal selected from the group of nickel, cobalt and iron form the catalyst.